Refactoring + fix time_bnds call + add out_names tests

[glevava]

Here is my new PR with the following changes:

• Fix the time boudary call in a CF compliant way (see point 2 of PrePARE issues #332)
• Add absolute value function to fill value test (see point 1 of PrePARE issues #332)
• Add required tests on the variable name from the file when the same CMOR out_name corresponds to several CMOR entries in the corresponding table.
• Add a "out_names_tests.json" file to easily add new cases if needed for further DR tags.
• General refactoring simplifying code (e.g., "variable" is a now a string instead of a list, etc.)

This PR has been tested at IPSL on IPSL-CM6 production file, I haven't tested all the archive we have, I focused my tests on the files concerned by the "out_names_tests.json"
Feel free to reject the PR if the PR don't pass all your tests.

Thanks